In electrical impedance tomography, the electric conductivity of a human or animal body between a plurality of electrodes is determined and an optical view of the body is derived from this. A so-called electrode belt with a plurality of electrodes is arranged on the skin surface of the body for measuring the electric conductivity. An alternating current, for example, with a frequency between 10 kHz and 200 kHz, with an amplitude of a few mA, is applied between two electrodes each. The propagation of this alternating current is detected by the remaining electrodes in the form of the drop in voltage between the electrodes and is used as a basis for the calculation of a tomogram in an impedance tomograph. The precise measurement and amplification of the measured voltages requires a highly sensitive electronic unit.
Electrical impedance tomography is nowadays used, among other things, to monitor the lung function of ventilated patients, for example, patients in an intensive care unit. If a cardiac arrest occurs in such patients, a defibrillator is used to reactivate the heart by targeted current pulses. Voltages of up to 5 kV are applied to the patient for a short time in this case. The highly sensitive electronic unit of the electrical impedance tomograph is not configured for such high voltages. Since, however, there is usually also no time for all electrodes to be removed from the patient before the defibrillator is used, other protective measures must be used to protect the electronic unit of the impedance tomograph against the current pulse of the defibrillator. In addition, it is also necessary to automatically disconnect the impedance tomograph from the patient in order to prevent the impedance tomograph from draining too much energy from the current pulse of the defibrillator and the current pulse is possibly no longer strong enough to stop the heart from fibrillating or fluttering.
According to DE 20 2005 013 792 U1, it is known to provide a protective circuit for an EIT apparatus that has a securing mechanism between the electrode on the patient and the signal input at the EIT apparatus. A series connection of two resistors, which is located in front of the first amplifier stage of the EIT apparatus, is inserted into the securing mechanism. Immediately after the beginning of a defibrillation, the electrodes are rapidly separated from the EIT apparatus by means of the securing mechanism. The drawback is that the securing mechanism only functions starting from a certain energy level of the defibrillator pulse and because of its size, this securing mechanism can only be implemented on the EIT apparatus and not on the electrode belt.
An overvoltage protection component, which can be configured as an overvoltage protection lever for a medical apparatus on the body of a patient, is known from DE 10 2008 002 330 A1. In this case, the overvoltage protection component is connected to a medical apparatus having a bipolar interface with a first and a second input. The overvoltage protection component comprises a first and a second capacitor as conversion means, which are arranged downstream at the inputs. Arranged downstream are two Zener diodes, which are connected in series with opposite polarity and electrically connect both input lines. The overvoltage protection component is only provided at the interface at the medical apparatus and not directly on an electrode belt.
A protective circuit for an electrical impedance tomograph is known from DE 10 2005 041 385 A1. This protective circuit, which consists of a series-connected RC circuit with a capacitor and a resistor, together with a spark gap, protects the signal inputs of the impedance tomograph against voltages that are too high during the normal measuring operation. In this connection, the capacitor is used as the actual protection against the actual current pulse. So that the operability of the impedance tomograph is not compromised, the capacitor of the RC circuit must have at least an electric strength of 5 kV. In order to guarantee the electric strength, the capacitor must have a minimal size, which makes the capacitor the component of the protective circuit with the greatest overall installed size. The minimal size of the capacitor leads, however, to it being more or less impossible to implement the protective circuit directly in the electrode belt due to a lack of space. Since actually each of the electrodes of the impedance tomograph must be dually protected, the overall installed size for the protective circuit is further increased.
Since it is more or less impossible to implement the circuit directly in the electrode belt, this circuit is nowadays usually arranged directly in the impedance tomograph. However, all components, from the electrode up to the lines, which lead to the impedance tomograph, must thus have an electric strength of at least 5 kV. In addition, the components must all be configured such that there is no risk due to the defibrillator pulse for medical staff that is in the vicinity of the impedance tomograph. Hence, the components of the electrode belt are relatively complex in terms of manufacture and are expensive. In addition, because of the low voltages that would have to be detected by the electrodes, it would be advantageous to arrange amplifier components directly in the electrode belt. In the current solutions, these active amplifier components would likewise have to have an electric strength of at least 5 kV, so that they are not destroyed by the defibrillator pulse. Since the active amplifier components are otherwise configured to amplify voltages with an amplitude of a few mV or μV, it appears to be very difficult to guarantee an electric strength for a defibrillator pulse at the same time.